There are many cases wherein it is necessary to determine the structure of the sea bed in a limited region. One case is where piles are to be driven into the sea bed to support a construction project such as columns of an offshore hydrocarbon loading platform or of various types of structures that are to lie offshore. If the seabed below a certain depth is consolidated (firm and secure) then piles driven therein will remain stationary, while if piles are driven into soft subsea soil then the piles are not as secure and soil strength must be considered. There is also the costly refusal of the pile by the presence of boulders. One way to determine the condition of the sea bed is to produce samples of the sea bed using core drills, which range in diameter between about 5 centimeters (2 inches) and 30 centimeters (12 inches). Another way is to conduct insitu cone tests. In insitu cone tests, a cone containing sensors is driven into the seabed and seabed characteristics at that location are measured. These two methods will sometimes be collectively referred to herein as seabed penetration measurement, by a seabed data penetrator. Since offshore core drilling and insitu cone tests are expensive and difficult to conduct, only a limited number of locations are drilled or interrogated by a cone. This leads to uncertainty about the condition of the sea bed. For example, if the core sample shows rock material extending down from a predetermined depth, there is uncertainty as to whether the rock is bedrock or is part of a boulder, or is part of a discontinuous hard pan layer.
The seabed can take the form of soft sedimentary lenses, boulders and/or cobble stones, a glacial till (clay, sand, gravel, and boulders intermingled), hard pan (compacted clay soil), mud layers, gas hydrates and gaseous sediments, and frozen soil. Many of these seabed materials are of different conditions when lying in situ (in the sea bed) than when present in a core sample, as where liquid and/or gas escape and/or very fine particulates drop out of the core or the temperature changes. It is possible to analyze the seabed by acoustic (sonar and seismic) apparatus wherein the sound is directed at the sea bed and the echos are detected. The echos indicate the reflectivity, attenuation, back-scatter, and velocities of sound at selected frequencies in the materials, from which the characteristics of the sea bed can be estimated. The interpretation of such acoustic sea bed characteristics is a more reliable presentation of the spatial extend of the layers than from a core sample or insitu core test alone. Acoustic imaging can cover a much wider area and at lower cost. It can also provide for lateral confirmation of the physical core properties.